Temperature and firearm violence in four US cities: testing competing hypotheses.

Introduction: Firearm violence is a major public health issue in the USA. There is growing evidence that firearm violence is associated with higher ambient temperatures. The aim of this study was to test competing hypotheses that could explain associations between temperature and firearm violence: temperature-aggression theory and routine activities theory.

Methods: We examined associations between elevated daily temperatures and shooting incidents in four US cities: Chicago, Illinois; Cincinnati, Ohio; New York, New York and Philadelphia, Pennsylvania. Temperature was operationalised using two different measures: daily maximum temperature and deviations of the daily maximum temperature from 30-year averages. Generalised linear autoregressive moving average models related temperature to shooting incidence while controlling for seasonal effects.

Results: As maximum daily temperature deviates from the expected, there was an association with increased shooting incidents in all four cities (eg, New York: b=0.014, 95% CI=0.011 to 0.017). An interaction term created by multiplying daily maximum temperature by the daily difference of maximum temperature from a 30-year average was also found to have a positive association in all four cities (eg, New York: b=0.020, 95% CI=0.016 to 0.025).

Discussion: These findings accord with previous studies demonstrating a positive relationship between temperature and firearm violence and further support temperature-aggression theory as the primary causal mechanism.